Breakwaters are generally shore-parallel structures that reduce the amount of wave energy reaching the protected area. They are similar to natural bars, reefs, or near shore islands and are designed to dissipate wave energy. For breakwaters protecting harbors, the breakwater acts as a barrier to wave energy and often to direct alongshore sediment transport away from the harbor. For shore protection, offshore breakwaters provide a reduction in wave energy in the lee of the structure slowing the littoral drift, producing sediment deposition and a shoreline bulge or “salient” feature in the sheltered area behind the breakwater. Some alongshore sediment transport may continue along the coast behind a near shore breakwater.
There are various types of breakwaters. These include:
Headland breakwaters, a series of breakwaters constructed in an “attached” fashion to the shoreline and angled in the direction of predominant wave approach such that the shoreline behind the features evolves into a natural “crenulate” or log spiral embayment.
Detached breakwaters that are constructed away from the shoreline, usually a slight distance offshore. They are detached from the shoreline, and are designed to promote beach deposition on their leeside.
Single breakwaters that may be attached or detached depending on what they are being designed to protect. A single detached breakwater may protect a small section of shoreline. A single attached breakwater, may be a long structure designed to shelter marinas or harbors from wave action.
System breakwaters refer to two or more detached, offshore breakwaters constructed along an extensive length of shoreline.
Rubble mound jetties are often referred to as breakwaters. They are oriented shore-perpendicular and usually built as a pair at a natural inlet, to provide extension of a navigation channel some distance from the natural shoreline. These structures redirect the sediment transport away from the navigation channel and constrain the tidal flow in the channel in order to make an efficient channel that requires little maintenance for navigation compared to a natural inlet.
Breakwaters are typically constructed in high wave energy environments using large armor stone, or pre-cast concrete units or blocks. In lower wave-energy environments, grout-filled fabric bags, gabions and other proprietary units have been utilized. Typical breakwater design is similar to that of a revetment, with a core or filter layer of smaller stone, overlain by the armoring layer of armor stone or pre-cast concrete units.
Armor units conventionally constructed of concrete are typically used to protect rubble mound structures in relatively high wave environments or where stone armor is not readily available. Rubble mound structures include breakwaters, revetments, jetties, caissons, groins and the like. Coastal rubble mounds are gravity structures. Conventional armor units are heavy in order to prevent displacement or rocking from waves and currents.
Armor units are typically displaced by one or both of two dominant modes of structure failure. The first is displacement of the armor which leads to exposure and erosion of filter layers and subsequently the core. The second is armor breakage. The breakwater or revetment capacity will be significantly reduced if either of these two failure modes occurs and progressive failure of the structure made much more likely. The under layer (filter layer) is sized so as to not move under undamaged armor and to prevent interior stone (e.g., small quarry-run stone) from escaping.
A wave is described by its height, length, and the nature of breaking. The wave height is the dominant forcing parameter considered in designing armor units. Other parameters include wave length, water depth, structure shape and height, armor layer porosity, degree of armor interlocking, inter-unit friction, and armor density relative to the water.
It is known that waves exert forces on armor units in all directions. Slender armor units usually require steel reinforcement while more stout armor shapes do not. Adequate steel (rebar) reinforcement increases material costs by roughly 100% over un-reinforced concrete. Both steel and polypropylene fiber reinforcement have been used to provide about 10-20% increase in flexural tensile strengths for large armor units. The cost increase for the fiber-reinforced concrete equates to an equivalent percent increase in strength.
The advantages and disadvantages of various existing concrete armor units are generally described in the above-referenced U.S. Pat. No. 8,132,985.
For most armor units, it is difficult to achieve adequate interlocking when placing underwater. This is particularly true when the visibility is low and there are background waves during construction. For pattern-placed armor, it is virtually impossible to place them correctly with no visibility or when background waves are present. This condition is quite common. Achieving interlocking and a smooth under layer when there is low visibility and background waves is extremely difficult and the uncertainty has led to cost overruns and even breakwater failures.
Relatively slender armor units, and hollow blocks like the shed and cob, require high-cost moulds and are challenging to cast. Metal mould cost depends on the number of plates and complexity of the bends. Some armor unit moulds require 75-100 plates. Cubes require the fewest plates but have all the concrete concentrated in one mass. This produces high heat of hydration and potential thermal cracking. Tall moulds used for large armor units and hollow blocks also have potential for significant strength variations throughout the armor unit because the aggregate settles, compaction is greater at the bottom of the mould, and water rises when the concrete is vibrated during casting. High water-to-cement ratios and over-vibration, which can occur in poorly supervised construction, results in degraded armor units. For example, aggregate can concentrate in the lower portion of the unit while the upper portion has an abnormally high water-to-cement ratio yielding weaker concrete. In addition, complex shapes have horizontal or shallow sloping surfaces where water can pool in the mould, further reducing strength. The result is that tall complex shapes depend greatly on the quality of construction processes and can yield less than optimum strength.
The application dictates the appropriate armor unit. For shallow, clear water with insignificant background wave conditions, and waves under eight meters in height, most of the previously discussed armor units can be constructed and placed without difficulty. In these cases, an engineer chooses the least expensive unit that provides the prescribed reliability. However, for low visibility, high background wave conditions, or waves of eight meters or greater, the disadvantages of inexpensive existing armor units mean that construction of a duality structure is going to be difficult and expensive and may even be filled with uncertainty. Further, long slopes in armored configurations provide more opportunity for down-slope settlement and potential armor breakage or displacement as the interlocking is lost. Although cube armor units are relatively easy to construct, they do not interlock so maintenance costs are much higher than other designs and cube armor requires far more concrete than many other designs.
There is thus a need for a durable interlocking armor unit capable of random placement resulting in a stable configuration that has strong individual units while being relatively straightforward to fabricate. Each unit should have slender appendages to provide improved stability and wave energy dissipation yet be strong enough to prevent failure of any single unit. The Limit should be suitable for repair of existing slopes. It should be relatively simple to fabricate and lend itself to ready stacking for storage and shipping, thus reducing overall cost, as well as to emplacement in conditions not conducive to emplacing existing units.
The Armor Unit disclosed and claimed in the above-referenced U.S. Pat. No. 8,132,985 solves many problems of pre-existing designs.
As discussed above and in greater detail in U.S. Pat. No. 8,132,985, the challenge of designing an appropriate structure with armor units while providing the best value for the cost is a continual challenge both for the designer and the construction and engineering concern placing the units and executing on the design.
The improved cost-efficient armor unit of the present invention quite surprisingly provides excellent hydraulic stability, structural stability, packing density and other performance criteria while reducing the cost of the armor units of U.S. Pat. No. 8,132,985, and represents a significant advance in the art.
Quite surprisingly, the present invention is based upon the unexpected discovery that when the design of the armor units of U.S. Pat. No. 8,132,985 lack one or both of the end frusta, cost can be dramatically reduced while still providing provides excellent hydraulic stability, structural stability, packing density and other performance criteria